Vibrator device driven by at least one imbalance generator

ABSTRACT

A vibrator device having a housing with a centrally apertured base, two vibrator plates disposed at either side of said base and held and guided in parallel relationship with one another and said base by means of threaded bolts, opposingly working coil springs fitted between said vibrator plates and either side of said base, an imbalance generator fixedly held by one of said vibrator plates and extending through said opening in said base; the distance between said vibrator plates is adjustable by nuts threadedly secured to said bolts.

United States Patent Inventors Appl. No.

Filed Patented Assignee Priority VIBRATOR DEVICE DRIVEN BY AT LEAST ONE IMBALANCE GENERATOR 16 Claims, 10 Drawing Figs.

US. Cl 74/61, 198/220, 209/367 Int. Cl ..Fl6h 33/00, 865g 27/00, B07b 1/44 Field of Search 4/61;

Eugen Stahle Magstadt;

Walter Harlin, Stuttgart-Kaltental; Friedrich Ruckert, Stetten Flider, allot, Germany Oct. 2, 1969 June 8, 197 1 Robert Bosch GMBH Stuttgart, Germany Oct. 2, 1968 Germany 7 198/220 CA, DB; 209/3665,

Primary Examiner-Fred C. Mattern, Jr. Assistant Examiner-Wesley S. Ratliff, Jr. Att0meyEdwin E. Greigg ABSTRACT: A vibrator device having a housing with a centrally apertured base, two vibrator plates disposed at either side of said base and held and guided in parallel relationship a, p 11 a with one another and said base by means of threaded bolts, opposingly working coil springs fitted between said vibrator plates and either side of said base, an imbalance generator fixedly held by one of said vibrator plates and extending through said opening in said base; the distance between said vibrator plates is adjustable by nuts threadedly secured to said bolts.

PATENTEU JUN 8197! 35 324 SHEET 1 BF 3 IN VENI'ORS Eugen STAHLE Walter HA 7?! IN Friedrich ROCKERT fhe/r ATTORNtW VIBRATOR DEVICE DRIVEN BY AT LEAST ONE IMBALANCE GENERATOR BACKGROUND OF THE INVENTION This invention relates to a vibrator driven by at least one imbalance generator and is particularly adapted for use in a vibrating conveyor or vibrating sieve, the effective mass of which is coupled to a free mass (at least partially constituted by the driving motor) by means of opposingly operating preloaded springs which have progressive or degressive characteristics and which are inserted between two spring supports. The preload of said springs and thus the amplitude of vibration may be varied by changing the distance between said supports.

A vibrator of the aforenoted type wherein the imbalance generator and the spring supports are secured to the effective mass, while the free mass is disposed between the opposing working springs, is described in German Published application No. 1,171,815. This publication further teaches the use of the imbalance motor as a free mass.

Further, vibrators of the aforenoted type are known that are associated with two opposingly driven imbalance generators, wherein the free mass is coupled with the effective mass by means of opposingly working springs of identical progressive or degressive characteristics and wherein the spring constants and thus the amplitude of vibration may be set or adjusted by varying the distance between the spring supports.

The free mass in the aforenoted known vibrators is formed of two vibrator plates guided in parallel, adjustable relationship by a threaded bolt and of the two imbalance generators which are secured to a vibrator plate face remote from the springs.

The opposingly working springs in the aforenoted vibrator are arranged in two pairs and are, at their outer ends, secured to a vibrator plate, while at their inner ends, they engage a housing base which is disposed between the two vibrator plates and which is disconnectably affixed to the effective mass.

For setting or adjusting the distance between the spring supports (that is, the distance between the vibrator plates,) there is provided a spindle nut mounted on said threaded bolt and tumable by means of a handwheel; the frontal face of the nut engages the vibrator plate which carries the imbalance generatOl'S.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide an improved vibrator particularly for use in vibrating conveyors or vibrating sieves, wherein the driving and adjusting means are designed as a structural unit which may be easily mounted on the vibrating apparatus and which has a compact and low structure and which will not become top heavy even if -as it is customary with vibrating conveyors-it is disposed in an inclined manner.

According to the invention, the free mass of the vibrator is fonned of two vibrator plates guided in parallel, adjustable relationship by at least one threaded bolt and of at least one imbalance generator secured to one of said plates; at least one pair of opposingly working springs are at their outer ends attached to a vibrator plate forming a spring support, while their inner ends engage a housing base which is disposed between the vibrator plates and is disconnectably attached to the effective mass and, further, the center of gravity of the free mass is at least approximately disposed in the medial plane extending between the opposingly working springs.

The invention will be better understood and further objects as well as advantages will become more apparent from the ensuing detailed specification of several exemplary embodiments taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a side elevational view in section ofa first embodiment ofa vibrator having a single imbalance generator; FIG. 2 is a sectional view along line 1II I of FIG. I

FIG. 3 is a sectional side elevational view of a second embodiment of a vibrator having two imbalance generators;

FIG. 4 is a sectional view along line N4 of FIG. 3;

FIG. 5 is a sectional side elevational view of a third embodiment of a vibrator having two imbalance generators;

FIG. 6 is a schematic side elevational view of a gear drive for synchronizing two imbalance generators associated with a vibrator;

FIG. 7 is a partially sectional plan view of an imbalance generator shaft provided with two eccentrical weights and two fly discs;

FIG. 8 is an elevational view of one spring of an opposingly operating spring pair having an increasing pitch along its entire length;

FIG. 9 is an elevational view of one spring of an opposingly operating spring pair having a pitch that increases from both ends of the spring to the middle thereof and FIG. 10 is an enlarged, fragmentary view of one portion of a slightly modified structure according to FIG. 3.

DESCRIPTION OF THE EMBODIMENTS Turning now to FIGS. 1 and 2, there is shown a housing generally indicated at 1 including a base 2 and dependent perpendicular sidewalls 3 reinforced at their free end by offstanding flanges 4. Bolts 5, extending through flanges 4, attach the housing I to a rigid frame portion 6 of an apparatus, such as a vibrating conveyor (not shown.)

Spaced from either side of the housing base 2, there are disposed two rectangular vibrator plates 7 and 8 extending parallel to one another and interconnected by means of two threaded bolts 9 fixedly secured to vibrator plate and extending slidably through corresponding openings in base 2. Further, the bolts 9 extend slidably through openings in vibra tor plate 8 and are secured thereto by means of a nut 10 threadedly received on each bolt 9.

Both the upper and the lower face of the housing base 2 is engaged by eight coil springs 11, forming eight spring pairs. The two springs within each pair work opposingly and are axially aligned. One spring of each pair is inserted in a partially compressed, preloaded condition between the upper face of the housing base 2 and the lower face of vibrator plate 8, while the other spring of each pair is fitted in a partially compressed, preloaded condition between the lower face of the housing base 2 and the upper face of vibrator plate 7. Each spring 11 has only two to five effective turns.

By means of the nuts 10 threaded on the bolts 9, the distance between the vibrator plates 7 and 8 and thus the preload of the opposingly operating springs 11 is adjustable The nuts 10 may be immobilized by means of securing nuts (not shown) against accidental turning.

To the midportion of the lower face of vibrator plate 8 there is secured, by means of a baseplate 12' an imbalance generator 12, the shaft of which (not shown extends parallel to the vibrator plast 8 and perpendicularly to the long side thereof.

The housing base 2 and the vibrator plate 7 are provided with openings 13 and 14, respectively, into or through which the imbalance generator 12 projects.

A cover 15 is secured to the housing 1 for enclosing the vibrator components disposed externally of the housing I.

The two vibrator plates 7 and 8, the imbalance generator 12 and the two bolts 9, together with the nuts 10, form a free mass which, by means of the opposingly working springs 11, is coupled to the effective mass including housing I and cover 15.

The free mass is dimensioned so that its center of gravity lies in the medial plane separating the opposingly working springs from one another. It is thus achieved that the structural height of the housing I, together with the cover 15, is very small.

The vibrator is driven in the subcritical range of the resonance curve, and for the tuning the formula q=falfe l applies, wherein fa is the driving frequency and fe is the natural or own frequency.

In view of the fact that the amplitude of vibration is a function of the tuning and the latter, in turn, is the quotient of the driving frequency and the natural frequency, the amplitude (and thus the conveying speed ofthe vibrating conveyor) may he varied by setting or adjusting either the driving frequency or the natural frequency. The latter of the two possibilities is usually resorted to for a tuning of the vibrator prior to the start of its operation. Since, further, the own frequency of the vibrator system is a function of the stiffness of the coupling springs, the required own frequency may be varied or set by using coupling springs of preselected spring characteristics.

By using a plurality of parallel connected coupling springs each having the same length and the same spring charac' teristics, the value of the latter relating to the entire spring system may be set or varied in a simple manner by adding or removing one or more spring pairs. For a course tuning, the number of opposingly working springs is selected in such a manner that it is proportionate to the replaced mass of the vibrating system.

Coupling springs having nonlinear characteristics permit a fine tuning by changing their preload, that is, by varying the distance between the vibrator plates 7 and 8. Such a tuning may be perfonned even during operation, provided the nuts 10 on the threaded bolts 9 may be turned during vibration by means ofa wrench, handwheels, or an adjusting motor.

It is, however simpler to perform a tuning during operation by means of the first-named possibility, that is, by varying the driving frequency of the imbalance generator, because small changes in the driving r.p.m. thereof are sufficient to vary the amplitude of vibration (and thus the conveying velocity of the vibrating conveyor) from zero up to a maximum value. For changing the driving r.p.m. of the imbalance generators, regulator transformers or tyristors may be used.

By virtue of the resonance drive and the coupling of the free masses with the effective mass by means of opposingly working coupling springs, the imbalance generator instead of circular vibrations, produces vibrations of the shape of a slender ellipse, the major axis of which is oriented in the working direction of the apparatus (conveyor, sieve or the like,) that is, normal to the housing base 2.

By using eight parallel connected spring pairs as coupling springs, the distance between the vibrator plates 7 and 8 and thereby the height of the housing 1 and cover is less than if only one or two spring pairs were used with correspondingly larger spring characteristics and greater structural length.

Due to the higher durability of coil springs having a smaller gauge, the lifetime of the coupling springs is longer. By using coupling coil springs of only a few active turns, their capacity and extent of deflection is limited. This feature is advantageous in that the vibrating conveyor is less sensitive to loads and further, that during the starting of the vibrator, or in case of a more substantial load placed thereon, an undesirable transition into the critical range and thereby a decrease of the amplitude of vibration to zero is prevented.

The extent of insensitivity to loads may still be further increased by coil springs having a progressive rather than a uniform pitch. With such coupling springs, the vibrator may be advantageously driven in the subcritical range very close to the frequency of resonance.

Turning now to FIGS. 3 and 4, there is shown an embodiment designed for generating a more substantial jarring effect. These figures depict vibrator which is operated by means of two synchronized, but opposingly driven imbalance generators 16. The shafts 17 of the imbalance generators 16 are disposed parallel to one another so that the generated vibrations are normal to the plane containing the two shafts 17.

The two imbalance generators 16 are secured to opposed faces of an intermediate wall 18 with their bases 16' in a backto-back relationship. The intermediate wall 18 is perpendicular to the two parallel arranged vibrator plates 19, and is welded to the vibrator plate 19 dividing it into two halves. The provision of the intermediate wall 18 as a mounting means for the two imbalance generators 16 results in the advantage that the distance between the two shafts 17 may be held at a very low value.

The intermediate wall 18 is reinforced by two triangular support plates 21 welded and disposed normal to both the intermediate wall 18 and the vibrator plate 19. The distance between the two support plates 21 is slightly less than the width of the vibrator plate 19.

As may be seen from the description that follows, in other details the embodiment shown in FIGS. 3 and 4 substantially corresponds to the embodiment depicted in FIGS. 1 and 2.

A housing generally indicated at 22 formed of a rectangular planar base 23 and dependent, perpendicular sidewalls 24 is reinforced by flanges 25 extending from the free ends of sidewalls 24. The housing 22 is fixedly secured to a rigid frame 27 forming part of a vibrating conveyor or the like, by means of screws 26 passing through flanges 25. The two imbalance generators 16 as well as the separating wall 18 extend through a central opening 28 of housing base 23.

To the vibrator plate 19 there are secured two bolts 29 extending normal therefrom and, passing through corresponding openings in housing base 23, project through vibrator plate 20, to which they are secured by means of nuts 31 threadedly received by the free end portions of bolts 29. The vibrator plate 20, unlike the vibrator plate 8 of the precedingly described embodiment, has a central, upwardly bulging portion 20 which accommodates upper parts of the two imbalance generators 16 and separating plate 18.

The upper and the lower face of the housing base 23 are each engaged by 28coil springs 30, forming 28 spring pairs. The two springs within each pair work opposingly, and are axially aligned. One spring of each pair is inserted in a partially compressed, preloaded condition between the upper face of the housing base 23 and the lower face of vibrator plate 20, while the other spring of each pair is fitted in a partially compressed, preloaded condition between the lower face of the housing base 23 and the upper face of vibrator plate 19.

By means of the two nuts 31 threadedly mounted on bolts 29, the distance between the vibrator plates 19 and 20, and thus the preload of springs 30 may be adjusted.

A cover 32, having sidewalls flush with the sidewalls 24, is secured to the housing 22 and encloses those components which are disposed above the housing base 23.

The two vibrator plates 19 and 20, the two imbalance generators I6, and the two threaded bolts 29, together with the nuts 31, form a free mass which is coupled with the effective mass (including the housing 22 and the cover 32) by means of the opposingly working springs 30. The center of gravity of the free mass is disposed in the medial plane separating the opposingly working springs from one another.

The driving means for the vibrator and the structural design of the coupling springs are identical to those described in connection with the embodiment shown in FIGS. 1 and 2.

FIG. 5 shows a further embodiment which, similarly to the structure illustrated in FIGS. 3 and 4, is associated with two imbalance generators. The embodiment shown in FIG. 5 differs from that of FIGS. 3 and 4 only as follows:

The mutually parallel shafts 33 of the two imbalance generators 34 are not normal but parallel to the long side of the rectangular vibrator plates 35 and 36. The two imbalance generators 34 are, by means of their baseplates, directly secured to the vibrator plate 35.

The two vibrator plates 35 and 36 disposed at either side of base 37 of housing 38 are interconnected by means of a single threaded bolt 39 carrying a nut 40. The bolt 39 passes through the point of intersection of the two diagonals of vibrator plate 35 and thus disposed in the center thereof.

The vibrator plate 36, for its positioning and guiding, is provided with a central bore 42 having an upper conical countersunk portion 41 receiving a complemental conical face of nut 40. This assembly ensures that the threaded bolt 39 is held centrally in, and without play with respect to, the vibrator plate 36. It is to be understood that the aforenoted engaging faces may be spherical rather than conical.

The aforedescribed guiding means for the vibrator plate 36 is advantageous in that an accurate fit between the bolt 39 and the cylindrical bore 42 is not necessary and that despite the wear of the contact faces between nut 40 and the vibrator plate 36 due to the substantial forces exerted by the opposingly working springs 43 and by the vibrations, an exact central guiding of the vibrator plate 36 may be maintained. Consequently, displacements of the vibrator plates 35 and 36 with respect to one another are avoided. It is to be noted that by deepening the countersunk portion 41, the length of the bolt 39 may be advantageously decreased.

By arranging the shafts 33 of the imbalance generators 34 parallel with the long side of the vibrator plates 35 and 36, undesired tilted oscillations of the rectangular plates in their lon gitudinal direction may be avoided by virtue of the gyroscopic effect ofthe rotating masses ofthe imbalance generators 34.

The aforenoted gyroseopic effect may be amplified by flywheels 50 (FIG. 7) attached to the shaft 51 of the imbalance generators 34 and having a centrally located center of gravity. If conventional imbalance generators are used in which an imbalancing mass is secured to the shaft ends, there is attached a fly disc 50 adjacent such imbalancing mass 52.

For synchronizing two imbalance generators associated with a vibrator device (FIGS. 3-5), there are generally provided two identical, meshing gears each secured to the parallel disposed shafts of the imbalance generator. In such an arrangement the diameter of the dividing circle of each gear is identical to the distance between the shafts of the two imbalance generators.

In conventional imbalance generators, because of legs or baseplates extending beyond the housing, the distance between the shafts and thus the diameter of the gears is relatively large, so that openings have to be provided in the housing base of the vibrator. In addition, the overall height of such vibrators (housing plus cover) is relatively great.

Turning now to FIG. 6, there is shown a gear assembly ineluding two gears 45 secured to the respective shafts of the imbalance generators 44 and two intermediate gears 46 having the same dimensions as gears 45. The intermediate gears 46 are held by a bearing plate 48 secured either to the two imbalance generators 44 or to the vibrator plate 47 carrying the imbalance generators 44. For a setting of the proper gear backlash, the bearing plate 48 may be displaceable by virtue of slots 49 provided therein.

In the aforedescribed gear assembly, the diameter of the dividing circle of each of the four gears in only one-third of the distance a between the shafts of the two imbalance generators 44, or only one-third of the diameter if only two gears are used. It is apparent that the insertion of the two intermediate gears 46 does not affect the rotational direction of the two imbalance generators 44.

The provision of the aforenoted intermediate gears 46, in the first place, permits the omission of openings in the vibrator plate 47, and, in the second place, results in a more compact structure of the vibrator device.

In FIG. 8 there is shown one of the opposingly operating springs formed as a coil spring 53 having only a small number of turns. Its pitch 54 increases end-to-end (downwardly, in this instance) by a constant value per each turn. A pitch increase of percent of wire thickness per turn was found to be advantageous.

The coil spring 55 illustrated in FIG. 9 differs from spring 53 merely in that the pitch 56 increases from both ends of spring 55 until the middle thereof.

FIG. 10 shows a portion of a vibrator device including a pair of opposingly operating springs 30. This vibrator device differs from the one shown in FIG. 3 only in that the ends of each spring 30 surround a centering projection 57 affixed to one face of each vibrator plate 19,20 and to both faces of housing base 23. By virtue of centering projections 57, a lateral displacement of springs 30 is prevented.

That which we claimed is:

1. In a vibrator device of the type driven by at least one imbalance generator and secured to an apparatus to be vibrated, the effective mass of the latter being connected to a free mass by opposingly working preloaded springs disposed between two spring supports, said free mass includes said imbalance generator, the preload of said springs being variable by changing the distance between said supports, the improvement comprising,

A. a housing disconnectably affixed to said effective mass and including a base having opposed faces,

B. first and second vibrator plates disposed parallel to one another adjacent to, and spaced from the opposed faces of said base and constituting said supports and forming part of said free mass, said at least one imbalance genera tor being affixed to said first vibrator plate,

C. at least one bolt passing through said base and connecting said vibrator plates with one another and adjustably determining the distance therebetween and permitting displacement thereof towards said base, said bolt forming part of said free mass, and

D. at least one spring pair consisting of a first and second individual spring, said first individual spring being fitted in a preloaded condition between one face of said base and said first vibrator plate, urging the latter away from said base, said first vibrator plate, urging the latter away from said base, said second individual spring being fitted in a preloaded condition between the outer face of said base and said second vibrator plate, urging the latter away from said base, the center of gravity of said vibrator device being disposed approximately in a medial plane extending between said first and said second individual spring.

2. An improvement as defined in claim I, wherein said base is provided with an opening and said at least one imbalance generator is disposed between said vibrator plates extending through said opening.

3. An improvement as defined in claim 2, wherein the imbalance generators are two in number and each includes a drive shaft, said imbalance generators are so disposed that their drive shafts are parallel to one another, said second vibrator plate includes an outwardly bulging portion to accommodate parts of said imbalance generators.

4. An improvement as defined in claim 3, wherein said vibrator plates are rectangular; said drive shafts extend parallel to the longer sides of said vibrator plates.

5. An improvement as defined in claim 3, wherein each imbalance generator includes fiywheel discs secured to said drive shaft, said flywheel discs are disposed adjacent, and have a mass substantially equal to the imbalancing weights of each said imbalance generator.

6. An improvement as defined in claim 3, including two gears, one keyed to the drive shaft of each imbalance generator, two intermediate gears meshing with one another and with the first named gears, bearing plate carrying said intermediate gears and secured to said first vibrator plate.

7. An improvement as defined in claim 1, including at least one nut having a centering face and threadedly received by said bolt, a bore in one of said vibrator plates adapted to slidably receive said bolt, a countersunk portion provided in said bore and having a face complemental with said centering face, the last named two faces are in engagement with one another. t

8. An improvement as defined in claim 1, wherein said individual springs are short coil springs having 2-5 turns.

9. An improvement as defined in claim I, wherein said individual springs are coil springs, the pitch of at least one of them varies uniformly from turn to turn.

10. An improvement as defined in claim 9, wherein the uniform variation of said pitch is approximately 15 percent of the wire thickness of the individual spring.

11. An improvement as defined in claim 1, wherein said individual springs are coil springs, the pitch of at least one of them increases from one end of the spring to the other end thereof.

12. An improvement as defined in claim 1, wherein said individual springs are coil springs, the pitch of at least one of them increases from both ends to the middle thereof.

13. An improvement as defined in claim 1, wherein said individual springs are coil springs; said opposed faces of said base, and said first and second vibrator plates are provided with centering projections each surrounded by at least the terminal turns of each spring to prevent displacement of the latter with respect to one another.

M. An improvement as defined in claim 1, wherein said 10 spring pairs are at least two in number and are disposed in symmetrical arrangement at either side of said at least one imbalance generator.

15. An improvement as defined in claim 1, wherein said opposingly working preloaded springs have progressive characteristics.

16. An improvement as defined in claim 1, wherein said opposingly working preloaded springs have degressive charactcristics. 

1. In a vibrator device of the type driven by at least one imbalance generator and secured to an apparatus to be vibrated, the effective mass of the latter being connected to a free mass by opposingly working preloaded springs disposed between two spring supports, said free mass includes said imbalance generator, the preload of said springs being variable by changing the distance between said supports, the improvement comprising, A. a housing disconnectably affixed to said effective mass and including a base having opposed faces, B. first and second vibrator plates disposed parallel to one another adjacent to, and spaced from the opposed faces of said base and constituting said supports and forming part of said free mass, said at least one imbalance generator being affixed to said first vibrator plate, C. at least one bolt passing through said base and connecting said vibrator plates with one another and adjustably determining the distance therebetween and permitting displacement thereof towards said base, said bolt forming part of said free mass, and D. at least one spring pair consisting of a first and second individual spring, said first individual spring being fitted in a preloaded condition between one face of said base and said first vibrator plate, urging the latter away from said base, said first vibrator plate, urging the latter away from said base, said second individual spring being fitted in a preloaded condition between the outer face of said base and said second vibrator plate, urging the latter away from said base, the center of gravity of said vibrator device being disposed approximately in a medial plane extending between said first and said second individual spring.
 2. An improvement as defined in claim 1, wherein said base is provided with an opening and said at least one imbalance generator is disposed between said vibrator plates extending through said opening.
 3. An improvement as defined in claim 2, wherein the imbalance generators are two in number and each includes a drive shaft, said imbalance generators are So disposed that their drive shafts are parallel to one another, said second vibrator plate includes an outwardly bulging portion to accommodate parts of said imbalance generators.
 4. An improvement as defined in claim 3, wherein said vibrator plates are rectangular; said drive shafts extend parallel to the longer sides of said vibrator plates.
 5. An improvement as defined in claim 3, wherein each imbalance generator includes flywheel discs secured to said drive shaft, said flywheel discs are disposed adjacent, and have a mass substantially equal to the imbalancing weights of each said imbalance generator.
 6. An improvement as defined in claim 3, including two gears, one keyed to the drive shaft of each imbalance generator, two intermediate gears meshing with one another and with the first named gears, bearing plate carrying said intermediate gears and secured to said first vibrator plate.
 7. An improvement as defined in claim 1, including at least one nut having a centering face and threadedly received by said bolt, a bore in one of said vibrator plates adapted to slidably receive said bolt, a countersunk portion provided in said bore and having a face complemental with said centering face, the last named two faces are in engagement with one another.
 8. An improvement as defined in claim 1, wherein said individual springs are short coil springs having 2-5 turns.
 9. An improvement as defined in claim 1, wherein said individual springs are coil springs, the pitch of at least one of them varies uniformly from turn to turn.
 10. An improvement as defined in claim 9, wherein the uniform variation of said pitch is approximately 15 percent of the wire thickness of the individual spring.
 11. An improvement as defined in claim 1, wherein said individual springs are coil springs, the pitch of at least one of them increases from one end of the spring to the other end thereof.
 12. An improvement as defined in claim 1, wherein said individual springs are coil springs, the pitch of at least one of them increases from both ends to the middle thereof.
 13. An improvement as defined in claim 1, wherein said individual springs are coil springs; said opposed faces of said base, and said first and second vibrator plates are provided with centering projections each surrounded by at least the terminal turns of each spring to prevent displacement of the latter with respect to one another.
 14. An improvement as defined in claim 1, wherein said spring pairs are at least two in number and are disposed in symmetrical arrangement at either side of said at least one imbalance generator.
 15. An improvement as defined in claim 1, wherein said opposingly working preloaded springs have progressive characteristics.
 16. An improvement as defined in claim 1, wherein said opposingly working preloaded springs have degressive characteristics. 